This invention relates to a high gain amplifier, and more particularly to such an amplifier which limits the current available when an intermediate stage is overdriven to positive feedback.
In certain types of high gain amplifiers such as disclosed in a co-pending patent application filed on even date herewith, entitled HIGH GAIN AMPLIFIER WITH RAIL TO RAIL RANGE AND FREQUENCY COMPENSATION, A. Paul Brokaw, which is an improvement on U.S. Pat. No. 4,857,862 (both of which are incorporated fully herein by reference), a second output gain stage is added to permit rail to rail response. In these types of amplifiers the input stage, a transconductance amplifier such as a differential amplifier, keeps the internal feedback balanced between positive feedback and negative feedback which is the reason for the very high gain of these amplifiers. However, upon overdriving the input the internal feedback can become strongly positive or negative depending on the direction of overdrive. In the case of positive feedback excessive power supply current can result in overloading the supply or damaging the circuit.
It is therefore an object of this invention to provide an improved high gain amplifier which limits the current that can flow in positive feedback conditions.
The invention results from the realization that a high gain amplifier whose input stage is balanced between positive and negative feedback in normal operation can be protected against excessive current levels when the input stage is overdriven into positive feedback by a limiting resistance which limits excessive current flow in positive feedback conditions but permits proper current flow in normal operation.
This invention features a high gain amplifier including an intermediate gain stage; an output gain stage driven by the intermediate gain stage, and an input stage, for driving the intermediate gain stage, which is balanced between positive and negative feedback in normal operation. There are bias means for driving the input stage to maintain balance between positive and negative feedback in normal operation. A resistance limits the output current of the intermediate stage in response to the input stage being overdriven into positive feedback.
In a preferred embodiment the output gain stage may include an inverting amplifier. The output gain stage may include a transistor amplifier. The resistance may be in the range between the supply voltage divided by the permitted current in the overdriven positive feedback condition, and a voltage comparable to the base to emitter voltage of the transistor amplifier of the output gain stage divided by the normal base current for the output stage. The resistance may be in series with or disposed between the intermediate stage and the output stage or between the intermediate stage and the bias means. The transistor amplifier may include a common emitter connected transistor The output of the output transistor amplifier may be at its collector. The intermediate stage may include a transistor amplifier. This transistor amplifier may include a common emitter connected transistor and the output of that transistor may be at its emitter. The bias means may include a current mirror. The input stage may include a pair of common emitter connected transistors. The input stage may also include a pair of input circuits for receiving input signals. The input circuits may be connected to the bases of the transistors. The resistance may be equivalent to the ratio of the supply voltage to the permitted current in the overdriven positive feedback condition. The resistance may be in series with the intermediate stage and the output stage.